1. The present invention relates to an electrophotographic image forming apparatus in which the distance between the photosensitive unit and any other unit associated with the photosensitive unit can be maintained at a constant value, and also to a method of positioning the photosensitive unit and other units of an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus comprises a photosensitive unit, a charging unit, a exposure unit, a developing unit, an image transfer unit, a fixing unit, a discharging unit, and a cleaner. The photosensitive unit has a photosensitive member or a photosensitive drum. Every time the photosensitive drum is rotated through 360.degree., the apparatus forms an image. More specifically, the charging unit electrically charges the photosensitive surface of the drum. Then, the optical image generated by the exposure unit is applied to the photosensitive surface, thus forming an electrostatic latent image thereon. Next, the developing unit applies toner onto the photosensitive surface of the drum, thereby developing the latent image into a toner image which is visible. In other words, the image is developed. Thereafter, the image transfer unit transfers the toner image from the drum to a sheet of recording paper. Then, the fixing unit fixes the toner image on the recording paper. After the transfer of the image, the discharging unit electrically discharges the photosensitive surface of the drum, and the cleaner removes the residua toner from the photosensitive surface of the drum.
The developing unit has a rotatable sleeve by which the toner particles carried by a toner cartridge are applied onto the photosensitive surface of the drum. To attract the toner, which has been applied from the developing sleeve, onto the photosensitive surface, it is required that a gap be provided between the developing sleeve and the photosensitive drum. This gap must be uniform all along the drum, so that the toner is uniformly distributed on the entire surface of the drum thereby to from a high-quality image. However, it is difficult to provide a gap which is uniform over the entire length of the drum, for the reason set forth below.
The photosensitive unit has a frame, in which the drum is rotatably supported. Similarly, the developing unit has a frame, in which the developing sleeve is rotatably supported. The frames of both units are attached to a support which in turn is fastened to the housing of the image forming apparatus. The frames are located such that the sleeve faces the drum contact. Annular protrusions are formed on the ends of the sleeve, thus defining a gap between the sleeve and the drum. Usually, the developing unit is positioned adjacent to the photosensitive unit by its own weight, so that the protrusions of the sleeve contact the photosensitive surface of the drum, with the gap maintained between the sleeve and drum.
In the above prior image forming apparatus, when the photosensitive drum is replaced with a new one, or cleaned, the developing unit is moved away from the photosensitive unit, to separate the developing sleeve therefrom. Thereafter, the new drum or cleaned drum is mounted on the apparatus. In this time, if the new drum or cleaned drum is mounted, mistakenly slantwise to the developing sleeve, the annular protrusions provided on both ends of the developing sleeve cannot properly come into contact with the photosensitive drum, so that the gap between the photosensitive drum and the developing sleeve is not uniform over the entire length of the drum. In this case, in order to obtain the uniform gap between the drum and the sleeve, the gap is adjusted. For the purpose of such gap adjustment, the photosensitive unit and the developing unit are mounted at the predetermined mounting positions of the housing by respective supports so that they can be slightly moved by respective adjusting members to finely adjust their mounting positions. In other words, the mounting positions of the photosensitive drum and the developing sleeve are finely adjusted by the adjusting members so that the gap between the photosensitive drum and the developing sleeve becomes constant over the entire length of the drum. However, it is difficult for a conventional operator to perform such fine adjustments as to make the protrusions of the sleeve uniformly contact the drum by slightly moving the drum and/or the sleeve so that the drum and the sleeve are in parallel with to each other. Accordingly, for example, when the operator replaces the photosensitive drum with a new one, and finely adjusts the photosensitive unit there is a possibility that and the developing unit, the operator will touch the photosensitive surface of the drum and damages it. For this reason, the replacement or cleaning of the drum must be performed by an expert service man.
The photosensitive drum and developing sleeve may be constructed with high accuracy so as to eliminate the need for fine adjustments. However, in this case, high accurate parts and high precision assembling are required for the drum and sleeve, resulting in an increased cost of the apparatus.
Even if the positions of the photosensitive unit and the developing unit are minutely adjusted before the image forming apparatus is delivered from the factory, these units are not always exactly parallel to each other, and the gap between them is not always uniform over the entire length of the drum. Further, even if such non-uniformity of the gap is tolerable, the gap between the, sleeve and the drum may become non-uniform to a greater degree due to the wear of the annular protrusions. When the gap between the sleeve and the drum is not uniform for whatever reason, it is no longer possible to distribute the toner evenly on the photosensitive surface of the drum, and a high-quality image can hardly be formed.